The constancy of living bodies, so-called homeostasis, is the essence of life phenomena, and among the phenomenon, the maintenance of intracellular pH is a most typical cell function. Presence of acid organelles, including lysosome as a typical example, in the cells with maintained homeostasis as described above is known for many years. It has recently been revealed that the acid organelles play important roles in various cell functions such as endocytosis, intracellular transport system, and autophagy. In order to correctly evaluate the functions of these acid organelles, a means for measuring pH, in particular, weak acidity (about pH 6) to moderate acidity (about pH 4 to 5), at specific sites where acid organelles exist in live cells has been desired. It has been attempted to develop a pH-sensitive fluorescent probe from viewpoints of high sensitivity, convenience of operation and the like. However, pH-sensitive fluorescent probes having been developed so far have a problem of decreased fluorescence intensity in an acidic region.
Various fluorescent probes for measurement of metal ions having an indacene structure have been proposed. For example, the compounds disclosed in Japanese Patent Unexamined Publication (KOKAI) No. 2005-022985 are known as fluorescent probes for zinc ion measurement having the indacene structure. However, the chemical structures of these compound are different from those of the compounds of the present invention in that they have at least one functional group for capturing zinc ion, and the aforementioned publication does not suggest nor teach that the aforementioned compounds can be used as a pH-sensitive probe. Compounds comprising the indacene structure bound with dimethylaminophenyl group are also known (Angew. Chem. Int. Ed. Engl., 36, pp. 1333-1335, 1997). However, the chemical structures of these compound are different from those of the compounds of the present invention in that they do not have carboxyl group.
Patent document 1: Japanese Patent Unexamined Publication (KOKAI) No. 2005-022985
Non-patent document 1: Angew. Chem. Int. Ed. Engl., 36, pp. 1333-1335, 1997